1. Field of the Invention
This invention pertains to the production of polyurethane foams through the use of delayed-action, heat-activated catalysts some of which release by-products at reaction temperature and foam the resulting polyurethane.
2. Description of the Prior Art
The reaction of polyisocyanates with polyols to form polyurethanes is a known process.
Activators or catalysts are used to increase the reaction rate of the polyisocyanate and the polyols. The usual commercial catalysts are amines (e.g., triethylenediamine), organo-metal compounds (e.g., stannous octoate) or some combination of the two. However, these catalysts are so effective that they detrimentally shorten the doctorability time in some processes (such as carpet foam-backing). To obtain a reasonable doctorability period with conventional commercial urethane catalysts, it is necessary to use a low catalyst level which causes the final cure to be slow and inadequate.
Various blowing or foaming agents are used, air, water, and low boiling inert liquids being the most common. Water reacts with the isocyanate groups, forming carbon dioxide. The low boiling liquids (e.g., trichlorofluoromethane) evaporate due to the exothermic polymerization reaction, causing foaming.
Erner et al. (U.S. Pat. No. 3,152,094) describe a class of imidazole catalysts that are unreactive at ambient temperatures, but effective catalysts at higher temperatures. These catalysts are represented by the formula ##STR2## wherein R.sub.2 is hydrogen, alkyl or benzyl; R.sub.1 and R.sub.3 are hydrogen, alkyl, benzyl, or joined to form a 6-membered carbocyclic ring, provided that the total number of carbon atoms in the substituents R.sub.1, R.sub.2 and R.sub.3 does not exceed 12.
Delayed-action, heat-activated catalysts are presented by McGraw et al. in U.S. Pat. No. 4,041,019. The catalysts correspond to the formula ##STR3## wherein n is an integer of from 1 to 3; R.sub.1 is an n-valent organic radical; R.sub.2 is hydrogen, hydrocarbyl or inertly-substituted hydrocarbyl; R.sub.3 and R.sub.4 are each independently hydrogen, hydrocarbyl, inertly-substituted hydrocarbyl, or, R.sub.3 and R.sub.4 are joined to form a 5- or 6-membered carbocyclic ring.
Wasilczyk, in U.S. Pat. No. 4,067,831 describes a process for preparing polyurethane foams. The foaming is done in the presence of a certain delayed-action catalyst. These catalysts are reaction products of various isocyanates with certain imidazoles. However, these delayed-action catalysts are used in addition to the conventional catalysts.